1. Field of the Invention
The present invention relates to a drive force control method for a four-wheel drive vehicle.
2. Description of the Related Art
In the case of turning a corner having a small turning radius in a four-wheel drive mode of a four-wheel drive vehicle in a low to medium vehicle speed range, a difference in rotational speed due to a difference in turning radius is generated between the front and rear wheels of the vehicle, causing a tight corner braking phenomenon. As the prior art for eliminating such a tight corner braking phenomenon, front and rear wheels driving devices are disclosed in Japanese Patent Publication Nos. 7-61779 and 7-64219.
The front and rear wheels driving devices disclosed in these publications have such a structure that a speed increasing device is provided between main drive wheels and auxiliary drive wheels to thereby adjust an average rotational speed of the auxiliary drive wheels to an average rotational speed of the main drive wheels. This speed increasing device includes a lockup clutch and a speed increasing clutch, which are selectively switched between ON and OFF states to thereby obtain a lockup condition where the average rotational speed of the main drive wheels and the average rotational speed of the auxiliary drive wheels are substantially equal to each other or a speed increase condition where the average rotational speed of the auxiliary drive wheels is greater than the average rotational speed of the main drive wheels.
Particularly in the front and rear wheels driving device disclosed in Japanese Patent Publication No. 7-61779, a torque distribution ratio between right and left rear wheels are controlled according to a vehicle speed and a steering angle so that the rear wheel torque is larger than the front wheel torque and the turning outer wheel torque is larger than the turning inner wheel torque. In this front and rear wheels driving device, the auxiliary drive wheels are increased in rotational speed by the speed increasing device in turning a corner having a small turning radius in the four-wheel drive mode, thereby preventing the tight corner braking phenomenon.
When the vehicle is accelerated during turning, the vertical loads on the inner wheels and the front wheels are reduced by the influence of lateral and longitudinal accelerations acting on the vehicle body. Further, since the front wheels are steered for turning, a lateral force acting on the front wheels is greater than that acting on the rear wheels. The greater the vertical load, the greater the drive force that can be generated by each tire. Therefore, the load on the tire of each front wheel is greater than the load on the tire of each rear wheel during turning at acceleration, and the load on the tire of each inner wheel is greater than the load on the tire of each outer wheel during turning at acceleration. The load on each tire depends on the degree of turning (the magnitude of lateral G) and the magnitude of acceleration.
It is effective to make the load on each tire uniform in improving the acceleration performance during turning. However, no mention as to making uniform the vertical load on each tire, or the load on each tire is made in the above publications, and minute drive force control is not disclosed in these publications. For example, a conventional drive force control method for the above front and rear wheels driving device has a problem such that the actual behavior of the vehicle body cannot be sufficiently grasped according to a road condition.